Polyesters are widely used in the manufacture of fibers, molded objects, films, sheeting, food trays, as well as food and beverage containers. These polymers are generally made by batch or continuous melt phase polycondensation reactions well known in the art. The polymers are then pelletized and used in various extrusion or molding operations. In certain applications where higher molecular weight polymers are required, the pellets are subjected to "solid state" polycondensation conditions in which the inherent viscosity (I.V.) value is significantly increased. Such solid state polycondensation reactions are used for two reasons. First, because the melt viscosity of polyester polymers is quite high for polymers having I.V. values greater than about 0.6, solid stating provides a convenient means to handle the polymer. Secondly, the solid stating process provides conditions conducive to removing unwanted volatile impurities such as acetaldehyde which is important in certain applications. Also, polyesters are well known to be degraded by small amounts of moisture when they are melt processed in conventional equipment. Consequently, polyesters are usually carefully dried to very low moisture levels in a drier prior to melt processing. The drying process may also remove some objectional volatile materials other than water.
During the preparation or processing of polyesters such as poly(ethylene terephthalate) (PET) certain byproducts are formed in the melt phase. One such byproduct is acetaldehyde, and its presence in molded objects such as food containers, beverage bottles, water bottles, and the like is quite deleterious from a taste standpoint. Particularly for sensitive beverages such as cola, beer, and water, it is highly desirable to produce container preforms for blow molding processes having less than about 10 ppm of acetaldehyde. Achieving this low level of acetaldehyde is difficult, however, because, as is well known to practitioners of the art, acetaldehyde is continually formed as a byproduct during the polymerization and subsequent melt processing of PET and similar polymers.
Before the discovery of the present invention, therefore, a four-stage process has been universally practiced to provide polyester polymers suitable for uses in which it is important to minimize the presence of acetaldehyde. Such a process typically involves the preparation of a relatively low molecular weight precursor polymer, having an I.V. value of about 0.3-0.6, by melt-phase polymerization techniques that are well known in the art. The acetaldehyde content of such a precursor may range from about 30 ppm to over 150 ppm, depending on the reaction conditions chosen. This precursor is then cooled, shaped into pellets, crystallized, and subjected to further solid-state polymerization at a lower temperature. Typically, a gas is used to strip glycols, acetaldehyde, and other reaction byproducts from the pellets so that at the end of the solid-state process, the I.V. value has been increased to about 0.75 or more, and the acetaldehyde content has been reduced to below about 1 ppm or less.
After solid stating, polyesters are commonly handled and stored in ambient air from which it absorbs moisture. Thus, as a third step, the polymer is usually dried immediately prior to reheating, melting and forming into a useful shape, such as a beverage bottle preform. The processing typically causes a small decrease in the I.V. of the polymer and an increase in acetaldehyde content of from less than 1 ppm in the pellets, to up to about 8 or 10 ppm or more in the shaped article. This dramatic increase in acetaldehyde occurs despite the fact that the molding process takes typically less than one or two minutes to complete.
U.S. Pat. Nos. U.S. 5,266,413, U.S. 5,258,233 and U.S. 4,8837,115 disclose various polyamides which are useful in reducing the acetaldehyde levels in PET. U.S. application Ser. No. 595,460, filed Feb. 5, 1996 discloses novel polyesteramide compositions which reduce the levels of acetaldehyde in PET.
Japan Patent Application Sho 62-182065 (1987) discloses blending nylon 6 and other aliphatic polyamides into PET to reduce the acetaldehyde levels below 10 ppm when residence times in the melt are kept below 60 seconds.
Several other compounds have also been disclosed to be useful in reducing acetaldehyde. These include ethylenediaminetetraacetic acid (U.S. Pat. No. 4,357,461), alkoxylated polyol (U.S. Pat. No. 5,250,333), bis(4-.beta.-hydroxyethoxyphenyl) sulfone (U.S. Pat. No. 4,330,661), zeolite compounds (U.S. Pat. No. 5,104,965), 5-hydroxyisophthalic acid (U.S. Pat. No. 4,093,593), poly(ethylene isophthalate) (U.S. Pat. No. 4,403,090) and supercritical carbon dioxide (U.S. Pat. No. 5,049,647 and U.S. Pat. No. 4,764,323) and protonic acid catalysts (U.S. Pat. No. 4,447,595 and U.S. Pat. No. 4,424,337).
U.S. Pat. No. 4,361,681 discloses that the capping of hydroxyl end groups of PET with anhydrides such as succinic or phthalic acid anhydride will suppress acetaldehyde formation. U.S. Pat. No. 5,243,020 discloses pyromellitic dianhydride for end capping of PET.
U.S. Pat. No. 4,356,299 discloses that the use of catalysts based on low levels of Ti and Sb are beneficial in limiting the amount of acetaldehyde formed.
U.S. Pat. No. 5,656,719 discloses the production of molded polyesters articles having low acetaldehyde via melt polymerization and a post condensation reactor. The post condensation reactor is operated at low vacuum levels and significant residence times to build up inherent viscosity.
U.S. Pat. No. 5,656,221 discloses the production of molded polyesters articles having low acetaldehyde via addition of an acetaldehyde reducing additive and molding directly from melt polymerization. Cobalt compounds in addition to conventional polymerization catalysts are required in amounts from 5 to 120 ppm.
Integrated processes which produce PET articles with low acetaldehyde but do not disclose the addition of acetaldehyde reducing additives have also been disclosed. U.S. application Ser. No. 609,197 describes a process for direct molding of polyester from the melt. U.S. application Ser. No. 498,404 discloses a process and apparatus for distributing molten PET to a multiplicity of molding machines. U.S. Pat. No. 5,648,032 describes processes for making low acetaldehyde poly(ethylene terephthalate) articles without using solid stated polymer.
Several patents disclose the use of vented extruders to devolatilize polymers. U.S. Pat. No. 5,597,891 discloses a process for producing reduced acetaldehyde polyester articles by using a purge gas in a vented extruder to remove acetaldehyde. U.S. Pat. No. 5,102,594 discloses thermoplastic condensation polymer supplied to extruder in powder form to a vented extruder). U.S. Pat. No. 3,486,864 discloses using a vacuum to remove volatile glycol products from a remelted prepolymer as fast as possible. U.S. Pat. No. 3,913,796 discloses using an extrusion screw for heating the solid resin to a semi-molten state and U.S. Pat. No. 4,060,226 disclose excluding oxygen by means of a check valve.